This is the final preprint version of a paper which appeared at Tbilisi Math. J. 1 (2008) 89-103. Published version accessible to subscribers at http://www.tcms.org.ge/Journals/TMJ/Volume1/Xpapers/tmj1_6.pdf On lattices and their ideal lattices, and posets and their ideal posets George M. Bergman 1 ∗ 1 Department of Mathematics, University of California, Berkeley, CA 94720-3840, USA E-mail: [email protected] Abstract For P a poset or lattice, let Id(P ) denote the poset, respectively, lattice, of upward directed downsets in P; including the empty set, and let id(P ) = Id(P )−f?g: This note obtains various results to the effect that Id(P ) is always, and id(P ) often, \essentially larger" than P: In the first vein, we find that a poset P admits no <-respecting map (and so in particular, no one-to-one isotone map) from Id(P ) into P; and, going the other way, that an upper semilattice P admits no semilattice homomorphism from any subsemilattice of itself onto Id(P ): The slightly smaller object id(P ) is known to be isomorphic to P if and only if P has ascending chain condition. This result is strength- ened to say that the only posets P0 such that for every natural num- n ber n there exists a poset Pn with id (Pn) =∼ P0 are those having ascending chain condition. On the other hand, a wide class of cases is noted where id(P ) is embeddable in P: Counterexamples are given to many variants of the statements proved. 2000 Mathematics Subject Classification. 06A06, 06A12, 06B10.. Keywords. Lattice of ideals of a lattice or semilattice; poset of ideals (upward directed downsets) of a poset; ideals generated by chains. 1 Definitions. Recall that a poset P is said to be upward directed if every pair of elements of P is majorized by some common element, and that a downset ∗The author is indebted to the referee for many helpful corrections and references. Archived at arXiv:0801.0751. Any updates, errata, etc. found after publication, will be noted at http://math. berkeley.edu/∼ gbergman/papers/ . Tbilisi Mathematical Journal (), pp. 1{15. National Centre for Science and Technology & Amsterdam University Press. 2 G. M. Bergman in P means a subset d such that x ≤ y 2 d =) x 2 d: The downset generated by a subset X ⊆ P will be written P # X = fy 2 P j (9 x 2 X) y ≤ xg: A principal downset means a set of the form P # fxg for some x 2 P: Definition 1.1. If P is a poset, an ideal of P will mean a (possibly empty) upward directed downset in P: The set of all ideals of P; partially ordered by inclusion, will be denoted Id(P ); while we shall write id(P ) for the subset of nonempty ideals. The subposet of Id(P ); respectively id(P ); consisting of ideals gener- ated by chains, respectively, nonempty chains, will be denoted ch-Id(P ); respectively ch-id(P ): This use of the term \ideal" is common in lattice theory, where an ideal of a lattice L can be characterized as a subset (often required to be nonempty) that is closed under internal joins, and under meets with arbitrary elements of L: For general posets, \ideal" is used in some works, such as [6], with the same meaning as here; in others, such as [13], \(order) ideal" simply means downset, while in still others, such as [8] and [5], an \(order) ideal" means a Frink ideal, which can be characterized as a directed union of intersections of principal downsets. (We shall not consider Frink ideals here. In upper semilattices, they are the same as our ideals. For a general study of classes of downsets in posets, see [5].) If S is an upper semilattice, its ideals are the closed sets with respect to a closure operator, so Id(S) is a complete lattice. If L is a lattice (or a downward directed upper semilattice), id(L) is a sublattice of Id(L); though not a complete one unless L has a least element. For S any upper semilattice, id(S) at least forms an upper subsemilattice of the lattice Id(S): In a poset P; the principal downsets (which we can now also call the principal ideals) form a poset isomorphic to P: If P has ascending chain condition, we see that every nonempty ideal is principal, so id(P ) =∼ P: (This yields easy examples where Id(P ) is neither an upper nor a lower semilattice.) The operators ch-Id and ch-id are not as nicely behaved as Id and id: Even for L a lattice, ch-Id(L) need be neither an upper nor a lower semilattice. For instance, regarding ! and !1 (the first infinite and the first uncountable ordinals), with their standard total orderings, as lattices, let L be the direct product lattice (! + 1) × (!1 + 1): (Recall that each ordinal is taken to be the set of all lower ordinals. Thus, ! + 1 = ! [ f!g and !1 + 1 = !1 [ f!1g:) Then the chains x0 = ! × f0g and x1 = f0g × !1 belong to ch-Id(L); but their join in Id(L); namely ! × !1; has no cofinal subchain (because ! and !1 have different cofinalities), hence does not lie On lattices and their ideal lattices 3 in ch-Id(L): Indeed, x0 and x1 have no least upper bound in ch-Id(L); since their two common upper bounds y0 = L # (! × f!1g) and y1 = L # (f!g × !1) intersect in the non-chain-generated ideal ! × !1: One also sees from this that the latter two elements, y0 and y1; have no greatest lower bound in ch-Id(L): Why are we considering these badly behaved operators? Because the method of proof of our first result involves, not merely an ideal, but an ideal generated by a chain, and it seemed worthwhile to formulate the result so as to capture the consequences of this fact. In a final section, x5, I shall note some variants of these constructions that are better behaved. We will also use Definition 1.2. A map f : P ! Q of posets will be called strictly isotone if x < y in P implies f(x) < f(y) in Q: Thus, the strictly isotone maps include the embeddings of posets, and so in particular, the embeddings of lattices and of upper semilattices. 2 Nonembeddability results. S. Todorcevic has pointed out to me that my first result, Theorem 2.1 below, is a weakened version of an old result of D.- Kurepa [11], which says that the poset of well-ordered chains in any poset P; ordered by the relation of one chain being an initial segment of another, cannot be mapped into P by a strictly isotone map. (A still stronger version appears in Todorcevic [15].) One could say that the one contribution of Theorem 2.1, relative to Kurepa's result, is that by weakening this assertion about chains to one about the downsets they generate, it gives us a statement about ideals of P: All the versions of this result have essentially the same proof; it is short and neat, so I include it. I give the result for posets; that statement implies, of course, the corresponding statements for semilattices and lattices. Theorem 2.1 (cf. [11, Th´eor`eme1], [15, Theorem 5]). Let P be any poset. Then there exists no strictly isotone map f : ch-Id(P ) ! P: Hence (weakening this statement in two ways) there exists no embedding of posets f : Id(P ) ! P: Proof. Suppose f : ch-Id(P ) ! P is strictly isotone. Let us construct a chain of elements xα 2 P; where α ranges over all ordinals of cardinality ≤ card(P ) (i.e., over the successor cardinal to card(P )); by the single recursive rule (1.1) xα = f(P # fxβ j β < αg): Given α; and assuming recursively that β 7! xβ (β < α) is a strictly isotone map α ! P; we see that P # fxβ j β < αg is a member of 4 G. M. Bergman ch-Id(P ); so (1.1) makes sense. We also see that for all β < α; the chain in P occurring in the definition of xβ is a proper initial segment of the chain in the definition of xα; so the strict isotonicity of f insures that xβ < xα; and our recursive assumption carries over to α + 1: It is also clear that if that assumption holds for all β less than a limit ordinal α; it holds for α as well. This construction thus yields a chain of cardinality > card(P ) in P; a contradiction, completing the proof. q.e.d. In the above proof, we restricted (1.1) to ordinals α of cardinality ≤ card(P ) only so as to have a genuine set over which to do recursion. The reader comfortable with recursion on the proper class of all ordinals can drop that restriction, ending the proof with an all the more egregious contradiction. Theorem 2.1 is reminiscent of Cantor's result that the power set of a set X always has larger cardinality than X: (Cf. the title of [9], where a similar result is proved with the poset of all downsets in place of the smaller poset of ideals.) In some cases, for instance when P is the chain of rational numbers, Id(P ) in fact has larger cardinality than P ; but in others, for instance when P is the chain of integers, or of reals, it does not.